test: refactor test suite for excluded paths and extensions

[appleboy]

• Update test function name to TestExcludedPathsAndExtensions
• Add test cases for excluded paths and extensions
• Modify router usage in test function
• Adjust assertions for content encoding, vary, body, and content length

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.35%. Comparing base (d3b1ccd) to head (0ae2960).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #80 +/- ## ======================================= Coverage 81.35% 81.35% ======================================= Files 3 3 Lines 118 118 ======================================= Hits 96 96 Misses 19 19 Partials 3 3 ``` | [Flag](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | Coverage Δ | | |---|---|---| | [go-](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [go-1.18](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [go-1.19](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [go-1.20](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [go-1.21](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [go-1.22](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [macos-latest](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | | [ubuntu-latest](https://app.codecov.io/gh/gin-contrib/gzip/pull/80/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib) | `81.35% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gin-contrib#carryforward-flags-in-the-pull-request-comment) to find out more.

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